Good news: this is actually working as intended 
Let me explain the reasoning, because it’s a deliberate design decision driven by two things:
- The macOS security model
- Keeping a simple mental model: sidebar = workspace, no sidebar = single-file editor
The first point is the important one. Lettera is distributed through the Mac App Store, which means it’s a sandboxed app: it can only access files you’ve explicitly given it permission to open (the one exception being Lettera’s iCloud folder, which we’re granted access to by default). So when you open a single file from Finder, that file is all we have access to, we can’t show the sidebar or its parent folder, because we simply don’t have permission to read them.
Your case is a bit special: the file lives inside the iCloud folder, so technically we could show the sidebar there. But then the app would behave differently depending on which file you double-click, and that’s confusing and hard to communicate. So we chose two clear, predictable modes instead:
- Open a folder → you get a workspace, with the sidebar.
- Open a single file → you get just the single-file editor.
Hope that clears it up! Let me know if anything’s still unclear.